The present invention relates to an internal floating roof located atop a fluid body in a fluid storage tank. Bulk fluids such as petroleum and other liquid fuel and chemical products are often stored in large cylindrical tanks. These tanks are commonly designed with internal floating roofs or covers to minimize product losses through leakage or evaporation to the atmosphere. The present invention describes a floating roof that is comprised of integral panels having a lower profile than existing floating roofs.
A large number of industrial processes require the use of substantial quantities of volatile liquids such as gasoline, alcohol, etc. The industries utilizing these processes store a wide range of volatile liquids in large storage vessels. The storage vessels are typically constructed of steel, stainless steel, aluminum and reinforced concrete, among other construction materials, depending upon the size and location of the storage vessel, the material stored inside the tank, and the industrial process generating or using the contained liquid. Many of these storage vessels have a fixed roof either integral with the vessel or retrofitted over the vessel for the dual purposes of keeping contaminants, e.g., water, dust and other particulate contaminants, out of the stored liquid and for reducing evaporative losses of the stored liquid for both economic and regulatory reasons. Storage vessels with a roof are commonly referred to as “covered” storage tanks.
If the liquid stored in the large-scale vessels is readily subject to evaporation at ambient pressure and temperature based upon their physical and chemical properties, additional control devices are commonly used to minimize losses from evaporation. Escaping vapors of many hydrocarbon based liquids can present health, safety or fire hazards. Vapors from flammable liquids can form an explosive mixture with air when an appropriate blend of stored liquid vapor and oxygen exists. Other liquids, particularly those containing sulfur, can present an objectionable odor when permitted to evaporate freely.
Over the years a variety of additional evaporative control devices have been utilized to control the escaping vapors from the liquids contained in the large-scale storage tanks. One common and effective variety of such control devices are liquid and vapor impervious buoyant structures that float on the liquid surface and are commonly referred to as “floating roofs.” If the storage vessel is covered with a separate structural roof, the floating roof is denominated as an “internal” floating roof. If the storage vessel does not have a roof or cover, the floating roof is denominated as an “external” floating roof. An external floating roof serves the dual purposes of keeping weather and airborne contaminants away from the stored liquid and in reducing evaporative losses.
Although many different types of floating roofs have been manufactured, most fit into two categories: vapor space and full contact floating roofs. Vapor space floating roofs typically contain a plurality of closed and sealed buoyant members for supporting an impervious membrane above the liquid surface. The buoyant members create a vapor space between the liquid surface and the underside of the impervious membrane. If any mechanical joints, seams or holes exist or are created through continued use in the membrane, liquid vapors from the vapor space below the membrane can leak through the membrane to the ambient atmosphere above the membrane creating a potentially hazardous atmosphere as well as an evaporative condition for the stored liquid. Full contact floating roofs are configured with the membrane in substantial contact with the surface of the stored liquid eliminating any vapor space below the membrane. Such full-contact membranes are typically the lower portion of closed and sealed buoyant members. While this is an improvement in creating a floating barrier for retaining the liquid in a non-evaporative state, thus controlling evaporation, there still exists the problem of mechanical joints, seams and holes that provide points of leakage. Additionally, creating and testing the closed and sealed buoyant compartments requires specialty materials, highly skilled designers and fabricators while testing and maintaining these compartments involves additional skills and work.
Existing designs for full contact floating roofs fall into two broad categories, i.e., monolithic and segmented. The present invention falls into the category of a segmented floating roof. Segmented floating roofs are typically fabricated off site and assembled within the storage tank. Each of the plural segments is typically comprised of a composite panel with edge closures that facilitate assembly one to the other. The composite panel is a structural component comprising an upper and a lower strong relatively thin metallic skin separated by and bonded to a lightweight edge material that creates a box-like form for the panel. Within the composite panel may be a core comprised of, for example, polyurethane foam or honeycomb aluminum to fill the void between the top and bottom skins and to assist in the buoyancy of the floating roof. The edge materials are connected together along their top and/or bottom edges with, for example, bolts and nuts or, for another example with a retaining hook along the bottom of a first panel for holding a distending flange of a second panel within the hook of the first panel as described in U.S. Pat. No. 5,704,509.
This description of a composite panel floating roof is provided to afford the reader with a reasonable understanding of the types of construction used in presently available floating roofs. However, there remain structural flaws that need to be addressed to further reduce evaporation, collection of volatile gases below and in the enclosed panel spaces, and reduce the vertical height to achieve less overall weight increasing the buoyancy and permitting greater storage capacity in the tank.
One of the noticed problems with the present designs for floating roof panels is the penetration through the hook and distending flange attachment between panels. This type of attachment arrangement permits the slow leakage (evaporation) of the contained liquid upward through any joint that is not rigidly held in absolute parallel to its adjoining edge member. Further, the hook may allow for some slippage away from the rigid joint through continued use. It is, therefore, an object of the present invention to eliminate the potential for slippage of adjoining panel edges away from one another by substituting a securing member for holding the edge joint in rigid contact along its entire length.
Another of the problems with the present designs for floating roof panels is the presence of fasteners in a potential vapor escape path that allow the passage of vapors to the space above the floating roof. The present invention eliminates the need for such fasteners or connections.
Another of the problems is the leakage of the liquid and/or vapors into the interior space of the composite panel creating a potentially hazardous condition and defeating the buoyancy characteristics for that panel. The present invention eliminates the top skin which, in turn, eliminates a potential collection space for harmful vapors in the core space of the panel. The present invention also eliminates the core material as the space between the edge members is now open to the ambient atmosphere. Thus, it is an object of the present invention to eliminate a collection space for harmful vapors by eliminating the upper skin and the core space. This, in turn, eliminates the need for buoyant core materials and allows for direct inspection of the bottom skin for leakage.
One other problem has been the additional buoyant members placed beneath the floating roof to maintain its buoyancy where required (typically at the outer edge of the floating roof where additional equipment is installed on top of the floating roof) and the subsequent loss of contact with the liquid surface. The buoyant members continually were in need of replacement as the liquids contained in the tanks seeped into them and destroyed their buoyancy. The present invention is a full contact floating roof that does not require additional buoyant members for floating support. It is another object of the present invention to eliminate the need to test and inspect the main and additional buoyant members for content and/or replacement. It is another object off the present invention to reduce the vertical profile of the internal floating roof and gain the efficiencies of lesser height increasing the potential volumetric capacity of the tank or container.
One additional problem is vapor leakage through the elongated mechanical seams between the edge members of the panels. Evaporative leakage is a problem as vapors can build up in the ambient atmosphere within the tank above the floating roof. If the seams are not absolutely tight, vapor can leak between the adjoining surfaces of the edge members even if they look as if there is no visible space therebetween. The present invention eliminates this source of leakage by placing a sealing means along the entire elongated surface of adjoining panel edge members. In this way leakage due to poor sealing between edges or due to panel warpage is eliminated.
Other objects will appear hereinafter.